As molding materials comprising a bundle of continuous reinforcing fibers and a thermoplastic resin as matrix resins, a wide variety of forms, such as thermoplastic prepregs, yarns and glass mats (GMT), are known. Such molding materials are characterized in that they are easily molded because of the properties of the thermoplastic resin; that they do not impose burden of storage as in the case of a thermosetting resin; and that they yield molded articles having high toughness and excellent recyclability. Particularly, those molding materials processed into the form of a pellet can be applied to a molding method having excellent economical efficiency and productivity, such as injection molding and stamping molding; therefore, such molding materials are useful as industrial materials.
However, for impregnating a bundle of continuous reinforcing fibers with a thermoplastic resin in the process of producing a molding material, there are problems in terms of the economical efficiency and the productivity; therefore, such a technique is not widely employed at present. For example, it is well known that impregnation of a resin into a bundle of reinforcing fibers becomes more difficult as the melt viscosity of the resin increases. A thermoplastic resin having excellent mechanical characteristics such as toughness and elasticity is a polymer having a particularly high molecular weight. As compared to a thermosetting resin, a thermoplastic resin has a higher viscosity and requires a higher processing temperature; therefore, it is unsuitable for producing a molding material with ease and good productivity.
Meanwhile, when a low-molecular-weight, that is, low-viscosity thermoplastic resin is used as a matrix resin because of the ease of impregnation, there is a problem that the resulting molded article has considerably inferior mechanical characteristics.
A fiber-reinforced composite material composed of reinforcing fibers and a matrix resin is light-weight and capable of providing excellent strength characteristics. Further, by controlling the fiber orientation, the resulting composite material can have an arbitrary strength design. Therefore, such fiber-reinforced material is widely used in, for example, sports applications such as golf club shafts and fishing rods; aerospace applications such as aircraft components and satellite parts; and general industrial applications such as automobiles, ships, electrical and electronic instruments, robot parts, windmills, tanks, bathtubs and helmets. Further, in the production of a fiber-reinforced composite material, a method in which a prepreg comprising reinforcing fibers impregnated with a matrix resin in advance is used as an intermediate substrate and laminated to produce a laminate is widely employed because the fiber content in the resulting laminate is generally easily increased and the laminate is relatively easy to hand. In prepregs, as a matrix resin to be impregnated into reinforcing fibers, a thermosetting resin such as an unsaturated polyester resin, a vinylester resin or an epoxy resin is often used because of the ease of impregnation into a fiber bundle. However, a thermosetting resin is cured to become an insoluble and infusible polymer having a three-dimensional mesh structure and such a polymer is difficult to recycle; therefore disposal thereof presents a more serious problem.
Meanwhile, as a thermoplastic matrix resin of a prepreg, a variety of resins such as polyethylene, polyester, polyamide and polycarbonate can be employed. In those applications where high performance is required, such as aerospace applications, polyether ether ketone, polyether imide, polyphenylene sulfide and the like, which are excellent in terms of the heat resistance, chemical resistance and mechanical properties, are preferably employed, and polyarylene sulfides such as polyphenylene sulfide are particularly preferably employed.
However, since such thermoplastic resin prepregs have a higher molecular weight as compared to that of a thermosetting resin, a high-temperature and high-pressure condition is required in the process of impregnating the matrix resin into a fiber bundle. A prepreg having a high fiber content is difficult to produce and there are 1.0 problems that, for example, the resulting prepregs are often non-impregnated prepregs and their mechanical properties are not satisfactory.
A fiber-reinforced composite material composed of a continuous reinforcing fiber substrate and a matrix resin is light-weight and has excellent mechanical characteristics. Therefore, such composite material is widely used in sporting goods, aerospace applications, general industrial applications and the like. Particularly, s composite material comprising a carbon fiber as a reinforcing fiber (CFRP) has a specific strength and a specific rigidity that are superior to those of metal materials; therefore, such composite material has been increasingly used primarily in aerospace applications. Conventionally, thermosetting resins have been preferably used as matrix resins because of their good impregnation into a reinforcing fiber base materials. A thermoplastic resin is a polymer having a high molecular weight and, as compared to a thermosetting resin, a thermoplastic resin has a higher viscosity and requires a higher processing temperature. Therefore, a thermoplastic resin is unsuitable for producing a fiber-reinforced molding base material with ease and good productivity.
In recent years, however, composite materials comprising a thermoplastic resin as a matrix resin have been drawing attention in various applications because they effectively shorten the molding time and molded articles obtained therefrom are advantageously recycled and have excellent post-processability for thermal adhesion, thermal reformation and the like. Among thermoplastic resins, a polyarylene sulfide has high elastic modulus and heat resistance as well as excellent fluidity. Therefore, a polyarylene sulfides can be suitably in a fiber-reinforced composite material from the standpoint of improving the mechanical characteristics of the resulting molded article. Accordingly, there is a demand for a more economical and productive method of producing a fiber-reinforced molding base material comprising a continuous reinforcing fiber substrate and a polyarylene sulfide.